(a) Technical Field
The present invention relates to a laser crystallization device and a laser crystallization method.
(b) Description of the Related Art
Liquid crystal displays (LCDs) and organic light emitting diode (OLED) displays, which are flat panel display devices, can be fabricated to be thin and light. Therefore, they are commonly used in mobile electronic devices and are increasingly used in large-scale display devices. As requirements for display devices with high speed operational characteristics emerge, research for such display devices is actively ongoing. In order to implement the high speed operational characteristics of a display device, a channel region of a thin film transistor (TFT) must be formed by using polycrystalline silicon instead of amorphous silicon. Amorphous silicon typically has electrical mobility of about 0.1 cm/Vsec to 0.5 cm/Vsec. In contrast, Polycrystalline silicon typically has electrical mobility of about 100 cm/Vsec to 300 cm/Vsec, thereby enabling a faster operational speed than amorphous silicon.
An annealing method using a laser is one of conventional methods for forming polycrystalline silicon. The annealing method involves irradiating a high energy laser beam to an amorphous silicon thin film deposited on a glass substrate. Upon receiving the laser beam, the amorphous silicon thin film is melted by the heat and then solidifies to be crystallized. This method is advantageous in that the glass substrate is not damaged by the heat.
In the laser crystallization method, energy distribution of an output laser beam, which generally has a Gaussian distribution, is changed by using an optical system. That is, in order to enhance uniformity and productivity of crystallization, the output laser beam is transformed into a linear laser beam, which is long in one direction, before being irradiated to the amorphous silicon thin film.
As a glass substrate to form the liquid crystal display is large, high laser output energy is required. Since the output of a light source is limited, a technique of combining the laser provided by a plurality of light sources has been developed. However, if the laser is combined, a speckle due to interference is generated such that a profile error portion is generated. If a laser beam profile is combined by using the same optical system, the profile error portion becomes even more serious. The profile error portion typically causes non-uniformity of the laser crystallization, which typically causes a display error of the display device.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention. The Background may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.